The Properties of Glass

Ordinary glass is colourless, and transparent to visible light rays, but opaque to ultra-violet and infra-red rays. The density varies from 2.25 for the lightest borate glasses to 6.35 for the heaviest lead and barium glasses. The average for alkali-lime glasses is 2.5 – 3.0. Glass is a bad conductor of heat, with the result that, when rapidly chilled, the outside cools long before the inside, producing a state of tension, so great in the more extreme cases that any disturbance of the surface, for example by scratching, may cause collapse of the glass to a fine powder with explosive violence. For this reason glass articles, before they are ready for use, must be annealed by heating for some time at a temperature which is fairly high, but not so high that deformation, or devitrification - that is, transformation into a brittle crystalline mass - is produced. The most suitable temperature for annealing varies with composition; for example, if the quantity of silica is kept constant the annealing temperature is reduced as alkali is substituted for lime. If lime be replaced by magnesium and aluminium, the annealing temperature is in general also lowered. An ordinary soda- lime glass has an annealing temperature in the neighbourhood of 600° C.

The coefficient of cubical expansion varies considerably, the mean value for soda-lime glasses being 0.000023 – 0.000027.

The change of properties with composition has recently been the object of much more systematic study than formerly. In particular, one might cite the long series of papers by Peddle on the development of various types of glass, and also the work by Turner and his colleagues on soda-lime glasses. Much work, however, still remains to be done. The general results hitherto obtained may be briefly summarised. Apparently contradictory conclusions have sometimes been reached, especially in the earlier work, but these are no doubt due to lack of clear definition of all the conditions.

Alkalies. - Soda and potash make glass more fluid and reduce the rate of setting. They diminish the resistant power towards water and acids, but increase it towards alkalies. Excess of soda especially increases the tendency to devitrification. A mixture of potash and soda produces a more durable glass than either alone.

Lime, substituted for soda up to about 20 per cent., diminishes the tendency to devitrification. It increases the chemical resistance, reduces the heat expansion, and increases the elasticity, rate of setting, and hardness. It makes the glass less fluid at low temperatures, but more fluid at high.

Lead oxide increases the density and refractive index. The glass is readily fusible, but unfortunately blackens when worked in the lamp. Lead glass is extraordinarily resistant to water, but not to acids and alkalies.

Barium oxide, substituted for lime, increases the density and refractive index, although not to so great an extent as lead; but it has the advantage over the latter of not blackening in the flame. It also increases the durability and decreases the solubility.

Zinc oxideis useful in glasses for laboratory and optical purposes. It gives a low coefficient of expansion to the glass, but a high tensile strength and chemical resistance.

Magnesia is similar to lime in its effect. It increases the viscosity and the heat-retaining power, which are advantages in working, but there is also a tendency to stringiness. Resistance to acids is slightly increased, but to alkalies it is diminished. Magnesia glass has a lower coefficient of expansion than lime glass, and also a lower annealing temperature.

Thallium oxide is sometimes used for special optical lenses. It makes the glass harder and increases the density and refractive index, but it is expensive.

Aluminium oxide. - The effect of aluminium oxide on glass has been the subject of much controversy in the past, but it now seems generally agreed that, up to a certain limit, the presence of alumina is beneficial. It reduces the tendency to devitrification, the thermal expansion, and the density, and increases the facility of working, although the effect produced depends greatly on the composition of the glass.

Silica gives high viscosity and chemical resistance. In very large quantity it causes devitrification and raises the temperature of fusion. Silica alone may be used as a glass, and has valuable heat-resisting properties; but a very high temperature, 1700°-1800° C., is necessary for fusion.

Boric acid gives low thermal conductivity and high chemical resistance if not present in too large quantity, for example, not more than 10 per cent., or even less. It was used in old Venetian glass. The coefficient of expansion, tendency to devitrification, and melting-point are lowered, whilst the refractive index is increased. Borosilicate glass is valuable for optical purposes.

Phosphoric acid up to a high percentage may be used in optical glass.

Small quantities of selenium, arsenic, and antimony are sometimes introduced to act as decolorisers and to help in fining the glass.

The durability of glass is determined by Mylius' weathering test. The glass is exposed to the action of water for a definite period, and the intensity of the pink colour developed in an ethereal solution of iodoeosin then noted. Another test, known as the " dimming test," has been suggested. The extent to which moisture is deposited on a carefully cleaned glass plate in an atmosphere of definite moisture content is observed, and the corroding effect afterwards studied under the microscope. A useful bibliography on the durability of glass is given by Turner.